Sensitive heat exchange detector



Feb. 25, 1958 M. e.-JAcoBsoN ETAL SENSITIVE HEAT EXCHANGE DETECTOR oliginal Filed A ril-1o, 1952 4 Sheets-Sheet l //v VEIVVTOYRSI Mos 6 6. JAcoaso/v, J34 MES c. 67L FER 7' FRANKUZ De}? (/CA WAL TER F. M an,

ATTOR EY Feb. 25, 1958 M. G. J coBsoN ETAL Re- 24,436

SENSITIVE HEAT EXCHANGE DETECTOR Original Filed April 10, 1952 4Sheets-Sheet 2 W R .5 m, MM!!! v K mwx AHZR,

a kw m x: EWT wMA m NARA Jrw I Feb. 25, 1958 JACOBSQN .ET Re. 24,436

SENSITIVE HEAT EXCHANGE DETECTOR Original Filed April 10, 1952 4Sheets-Sheet 3 W I INVENTO RS M0556 aJncaasalv,

JAMES c; 6/4 FER r, FRANK a Delve/a, WALTER F? MRU/f,

Feb. 25, 1958 M. G. JACOBSON ET AL 24,436

SENSITIVE HEAT EXCHANGE DETECTOR Original Filed April 10, 1952 4Sheets-Sheet 4 Iii-EB INVENTORS 4 M0556 6. JACUBSO/V, JAMES c. GILFERT IFRANK J? De 1. VGA

WALTER l-T Amok F E a BY United States Patent SENSITIVE HEAT EXCHANGEDETECTOR Moses G. Jacobson, Penn Township, Allegheny County, Pa., JamesC. Gilfert, Columbus, Ohio, and Frank J. De Luca, Pittsburgh, and WalterF. Mruk, McKees Rocks, Pa., by Mine Safety Appliances C0., Pittsburgh,Pa, assignee Original No. 2,769,884, dated November 6, 1956, Serial No.281,632, April 10, 1952. Application for reissue November 6, 1957,Serial No. 697,298

9 Claims. (Cl. 201-63) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

Our invention relates broadly to heat exchange measurements and moreparticularly to an improved construction of an electrical sensitive heatexchange detector.

One of the objects of our invention is to provide an electricallyoperated detector with improved heat exchange characteristics and inconsequence thereof with higher sensitivity.

Another object of our invention is to provide a sensitive heat exchangedetector which will not reflect all random temperature fluctuations asrapidly as they may occur, but will integrate them into an average valueover a small time interval congruent with the period of the indicatingmeter and the particular practical application.

A further object of our invention is to provide a detector element whosesensitivity will be largely independent of its angular position in spaceand relative to a container wall, which is especially important foranemometers, flowrneters, thermoconductivity meters, etc.

Still another object of our invention is to provide a construction of adetector element which is sturdy, rigid and of great durability andwhich will stand up under the severe conditions of use to which manyindustrial instruments are subjected, yet will have a sensitivity ashigh or higher than those heretobefore used only in laboratories.

Other and further objects of our invention reside in the compactconstruction and assembly of parts of a heat exchange device as setforth more fully in the, specification hereinafter following byreference to the accompanying drawings, in which:

Figure l is a longitudinal sectional view through one embodiment of ourinvention; Fig. 2 is an elevational view of the form of our inventionillustrated in Fig. 1; Fig. 3 is an enlarged fragmentary sectional viewshowing one end of the device illustrated in Figs. 1 and 2; Fig. 4 is atransverse sectional view taken on line 44 of Fig. 1; Fig. 5 is atransverse sectional view taken on line 55 of Fig. 1; Fig. 6 is alongitudinal sectional view taken through a modified form of ourinvention; Fig. 7 is an elevational view of the form of the inventionillustrated in Fig. 6; Fig. 8 is a transverse sectional view taken online 8-8 of Fig. 6; Fig. 9 is a transverse sectional view taken on line9--9 of Fig. 6; Fig. 10 is a longitudinal sectional view through anothermodified form of our invention, the view showing the high temperaturecoefii cient of resistance conductor in front elevation; Fig. 11 is aview similar to the view shown in Fig. 10, but illustrating theconductor in side elevation; Fig. 12 is an elevational view of the formof our invention shown in Figs. 10 and 11; Fig. 13 is a transversesectional view taken on line 1313 of Fig. 10; Fig. 14 is a transversesectional view taken on line 1414 of Fig. 10; Fig. 15 is a longitudinalsectional view taken through a further modified form of our invention;Fig. 16 is an elevational view of the form of our invention shown inFig. 15; Fig. 17 is; a transverse sectional view taken on line 17-17 ofFig. 15; Fig. 18 is a transverse sectional view taken on line 18-18 ofFig. 15; Fig. 19 is a perspective view showing. one of the end retainersemployed in the form of our in.-- vention shown in Figs. 15-18; Fig. 20is a theoretical view explaining the principle of our invention; Fig. 21shows.

an embodiment of our invention in a detector element employing theprinciple illustrated in Fig. 20 and which we have used to establishexperimentally the validity of; this principle; Fig. 22 illustrates oneof the practical em: bodiments of our invention in a heat sensitivedetector; Fig. 23 illustrates a further modified form of heat sensi-,

tive detector embodying our invention; Fig. 24 shows. still.

another form of heat sensitive detector constructed in accordance withour invention; Fig. 25 is a perspective view of a conically arrangedheat sensitive device embody-.

ing our invention for application as a flow indicator and anemometerelement; Fig. 26 is a transverse sectional. view on line 26-26 of Fig.25; Fig. 27 is a transverse sec.-

tional view on line 27-27 of Fig. 25; Fig. 28 is, a view showing aprojection of the detector element of Fig. 25 onto a plane perpendicularto the axis of the cone shown in Fig. 25; and Figs. 29 and 30 aretheoretical diagrams; explaining the theory of our invention.

Our invention is directed to the general class of in:

struments in which the electrical resistance of a conductor ofelectricity is changed by a variation in the temperature of theconductor caused by heat exchange. between it and a surrounding medium.This heat exchange may take the form of heat being carried away fromthe. v

conductor by a fluid moving past it as in anemometcrs or flowmeters orconvection indicators, or it may take, the form of heat being carriedaway by thermoconductivity of a surrounding gas or liquid as ininstruments for gas or vapor analysis by thermoconductivity. take theform of heat being absorbed by the conductor from the medium, which inturn is heated by combustion of one of its constituents or by absorptionof infrared or other radiation or from another source of heat located inthe medium as in calorimeters or Thomas meters. Or they heat exchangebetween the element and another body may take place solely by [omission]emission or absorption of radiant energy, as in spectrometers, opticalpyrometers, etc.

The simplest form in which such elements are applicable is a resistancethermometer; in this application as in many others conditions must be soarranged that the heat transferred from or received by the medium to orfrom the detector will not change the temperature of the medium to anyconsiderable extent, while changing the temperature of the detector tothe greatest possible extent. In general, in all these applications theefliciency of the heat transfer process is of major importance. Ourdevice improves these heat transfer conditions and provides a betterdetector element than has heretofore been known in the art.

Although, as hereinbefore pointed out, there are many applications forour heat exchange detector, the detailed description hereinafterfollowing will be given with ref erence to fluid flow and velocityindicators, for which specific purpose our preferred embodiment wasdeveloped.

Heretobefore, it has been customary to use for these detector elementsthin wires of materials with relatively high specific electricalresistances and with temperature coefficients as high as possible. Forthe material of these wires, platinum, tungsten and palladium were usedmost frequently. The elements are made up in most cases either in theshape of relatively long substantially straight. pieces of wire, or inthe form of small coils. Both of these forms are lacking very much instrength, especially when in order to obtain proper operation andsuflicient It may also sensitivity they have to be heated during theirnormal use to rather elevated temperatures. To provide greater strengthin some of the applications (as for instance in thermoconductivitycells), the wire surface is covered with thin layers of glass or enamel;and in other applications, especially in cases where the wire has to beused in or near the state of incandescence, it has been proposed to windresistance wire coils on rigid mandrels of some ceramic material, hardglass rods or the like. In both of these cases, the improvement inmechanical strength is obtained at the cost of considerable sacrifice insensitivity. Moreover, the poor thermal conductivity of the glasscovering, and especially of the ceramic mandrels, causes a veryconsiderable slowing down of the heat exchange between the medium andthe detector element and a slow creeping of the indications towards theequilibrium indication.

It has also been proposed to wind the resistance wire on a metal mandrelwith a layer of glass or other insulating material between them, and touse the mandrel itself as one of the lead wires, while this isundoubtedly better with respect to lag and creeping action than theceramic or mandrels of other insulating materials, nevertheless, all theforegoing composite detector constructions are directed to the obtainingof mechanically strong elements and so far as we are aware, theinfluence of such composite construction of these detector elements oneither the heat exchange conditions or on the electrical characteristicsor on the resulting sensitivity and time lag conditions has notheretofore been considered. Our investigation and experiments have shownthat these influences are very considerable, and by properly applyingthe results of our investigation, considerable improvements have beenobtained in the performance of heat exchange devices.

The basic'ideas of our improvement are as follows:

When an electric current I is passing through a wire of diameter d andlength I and having specific electric resistance p and temperaturecoefficient of resistance a, a quantity of heat Q1=- p[ o)]%, (1)

is developed in every unit of time, where T and T are the temperaturesof the wire and of the surrounding medium respectively. When temperatureequilibrium is reached-which is indicated by the fact that an electricmeter connected in shunt or in series or in a Wheatstone bridge circuitwith the heated wire is maintaining a constant defiectionthis quantityof heat developed is exactly balanced by the heat lost through thesurface of the wire by thermoconductivity, convection and rediation andin general also by thermal conductivity through the wire terminals [of]or the so called end losses. However, in the great majority ofapplications of this invention, the end losses are very small and to afirst approximation may be neglected. Substantially, then, all the heatloss takes place through heat exchange between the surface of the Wireand the surrounding medium. This heat lost can be expressed by theformula:

where h, which'we shall call the heat transfer constant, is the heatgiven off to (or in general exchanged with) the surrounding mediumthrough one unit of surface area; this quantity h is a function of thethermal characteristics (thermal conductivity, specific heat, etc.) ofboth the wire and the surrounding medium and of the condition of theboundary surface between the two; in the case of any relative motionbetween the medium and the Wire (as in anernometers and flowmeters), his also a function of the velocity of that motion. The function f(TTexpresses the temperature dependence of the heattransfer. When T thetemperature of the heated wire is not more than 80-1OG C. above thetemperature of the medium T this function can be adequately expressed byNewtons law of cooling, that is by a linear quantity of the form (TTwhere h is a constant coeflicient for a given wire and a given medium.For higher temperature differentials the reltaionship is no longerlinear, but is still a rising function of TT When equilibrium isreached, we have the following equation between heat developedQ and heattransferred-Q in every unit of time:

This equation relating to a single straight or a coiled filament hasenabled us to achieve a number of improvements in the development ofsingle wire filaments.

Let us now proceed a step farther and develop an equilibrium equationfor the case when-as in the present inventiona length of resistance wire1 of diameter d with a specific resistance p and high temperaturecoeflicient of resistance a is used not suspended freely in space andsupported only by two terminals, but is tightly wound around a mandrelwire of the same material of length L and diameter D, e. g. Let ussuppose for the time being that both wires are covered with a thin layerof the same insulating material. The heat transfer constant per unitarea and unit temperature differential will be the same for both theresistance wire and the mandrel. Also the temperature dependencefunction f(T T will be the same for both surfaces. The heat equilibriumequation, now will be:

2L 1 [111 1+a(T T 1rd TDIL L h (5) where r and m/n has a maximum valueof 1, when the convolutions of the resistance wire covers the entirelength of the mandrel without any interspaces. Comparing Equation 5 withEquation 4 we see that first of all, while in Equation 4 the totallength l of the wire cancels out, it does not cancel out in 5. Thatmeans that by winding the wire on the mandrel, We acquire anotherparameter which we can vary in order to improve the performance of thecomplete unit. To investigate the situation farther, we find the ratioof electrical currents 1 /1, which will give us equal temperaturedifferentials T1T =TTu From (6) we deduct that by choosing a mandrel ofsuch a diameter D that D becomes for instance nine times larger than thediameter of the resistance wire d, we can have the same temperature inour element with a more than three times larger electric current or withover nine times the electric power-provided the lengths L and 1 areequal. By making L larger than 1 we could increase this ratio stillfurther.

These detector elements are almost always used in Wheatstone bridgecircuits; to increase the electrical sensitivity it is necessary toincrease the voltage applied to the bridge and thus also the electriccurrent through the element; with elements made of single wires, therapid temperature increase with electric current puts severe limitationsto the electric currents that can be safely and practically used. Whilewith the resistance wire wound tightly on a heat conducting mandrel, wesee now that much higher electric currents can be used without gettingout of a safe temperature range.

In cases where it is desirable for reasons of current and voltageeconomy to obtain the same temperature range with smaller electriccurrentsas in many portable battery powered instruments-the newarrangement also offers considerable advantages: to make I smaller thanI for the same T'T all We have to do according to Equation 6 is to makethe ratio D L/d1 small-z while D cannot be made smaller than d, L can bemade so much smaller than 1 that D L becomes considerably smaller thand1.

Let us now go farther and investigate what might be called the thermalsensitivity of a composite detector element, and whether we do not losethe advantage gained from the increase in electric sensitivity by theincrease of the electric current by any possible decrease in the heattransfer characteristics; for example, in a detector element for afiowmeter, the question may arise whether the amount of cooling in unittime per unit area of surface and per unit temperature differential doesnot become smaller when we change from a single Wire element to a two ormultiple wire element. Our theoretical study shows that [L] h shouldremain substantially unchanged provided a small enough unit is chosenfor area measurement. As to the net total etfect of changing over from asingle wire element to a composite element of exactly equal surface areaand everything else the same, some changes can be expected due to thechange in the macrostructure of the surface: the single wire has an evengeometrical surface, While a composite element generally has a surfaceconsisting of small hills and dales; while in some applications, such asthermoconductivity detectors, this can be expected to favor the totalheat exchange; it will in other applications, such as in flowindicators, somewhat decrease it. Although accurate comparison tests onthis point are very diflicult, indirect observations have definitelyestablished that this effect is quite small, not more than 25%; whilethe gains obtainable by the composite construction from the increase inelectrical sensitivity are of the order of several hundred percent.

The conclusions drawn from the foregoing mathematical presentation showsthat a composite element consisting of a long and relatively thinresistance wire intimately joined together in thermal but not electricalcontact with another wire or body of high thermal conductivity providesnot only a mechanically stronger detector element, but also, by virtueof the greater increased surface available for heat exchange, providesthe possibility of greatly increasing the available electric power atthe same Working temperature. Returning to [Equation 5] Equations 4 and5 We see that this increase in electric power is made possible:

(a) By being able to use much higher electrical currents,

(b) By increasing the electrical resistance, by using resistance wiresof much greater length and smaller diameter without fear of them bulgingout or otherwise changing their shape or position or burning out fromaccidental overloads.

There is one more factor that favors the composite construction which Wehave not yet explained. It is always the desire of designers oftemperature sensitive resistance elements to use wire not only with thehighest possible temperature coefiicient but also of the highestpossible specific resistance i n order to have available the highestpossible electric power and by virtue of it the highest possiblesensitivity. But unfortunately with the increased specific electricalresistance always goes also increased thermal resistance or decreasedthermalconductivity; and thus a great deal of the gain by increase in pis lost because of the decreased thermoconductivity. In our compositeelement we use a resistance wire of the highest available specificresistance aswell as of very small diameter because the loss. in thermalconductivity caused; by this will be amply restored by the use of amandrel wire of a material of high thermal conductivity such as gold orsilver or copper. Thus by using for the mandrel, materials of highthermal conductivity instead of'the same material as in the resistancewireas was assumed temporarily hereinbefore for the sake of simplicity-afurther advantage is gained, which was not realized heretobefore.

As mentioned above, the heat transfer constant h in general is affectedalso by the specific heat of the material; however, when using metals onan equal volume basis this factor can be neglected; for, according tothe law of Dulong and Petit, specific heats of all metals are such thattheir molecular heat capacities (specific heat times molecular weight)are very nearly equal, and hence their thermal capacities when taken inequal volumes are very nearly equal. Therefore, in selecting materialsfor the composite detector elements, only the electrical resistancecharacteristics for the resistance Wire and only the thermalconductivity for the mandrels will be the determining factors.

in the great majority of applications of heat exchange instruments, avery fast response is not only not an advantage out very detrimental;for instance, an anemometer or airflow indicator with a response time of.1 second would be entirely useless for measurement of ventilationcurrents in mines and air conditioning [dusts;] ducts; for, even in awell regulated tunnel, it is diflicult to keep rapid fluctuations in theair velocity down to a few percent, and in actual air flow measurementsit is very seldom that these fluctuations do not amount to 10% or moreof the measured value. If an instrument even with a response time near asecond was used, the pointer of the indicating meter would fluctuate allthe time and make it quite difficult to make a reading. On the otherhand, if the response time is of the order of 5l5 seconds, a reading iseasily obtained, and as long as this response time remains constant,obviously, a consistent and true average indication is obtained. Incopending application Serial Number 41,347, filed July 29, 1948, byMoses G. Jacobson, now Patent 2,694,923, issued November 23, 1954, forElectrical System for Measuring the Rate of Motion of a Fluid, thevarious conditions and factors determining the time constant of heatresponsive detector elements are set forth, and a number of novelfeatures claimed to which reference is made for details of the timeconstant features of general interest herein. It will suflice to statehere, that as our experience has shown, composite detector elements ofthe construction described in this application can easily be made withtime constants of one second and higher. The time constants of thepreferred detector elements described hereinafter is of the order offive-six seconds.

Our invention may be embodied in various forms as set forth in detail inthe drawings which show the several forms approximately 20 to 50 timesenlarged with respect to the natural size of the actual units and thedimension of the insulation layers is exaggerated with relation to otherparts.

In Figs. 1 to 18 and 25 to 28 reference character 1 designates a Wire orelectric conductor of a relatively high electric resistance and with ahigh temperature coefficient of resistance; reference character 2designates terminals of high electric conductivity to which the two endsof the wire or conductor 1 are joined by soldering, welding or any otherknown means of securing a permanently good electrical connection.Reference character 3 designates a bar or wire made of a material ofhigh thermal conductivity, such as gold, silver, or copper. The numeral4 designates a thin layer of electrical insulation such as, forinstance, Glyptal, Formvar, or the like, the layer providing goodelectrical insulation while heated to temperatures of 159 C., yet retaintheir flexibility and low resistance to heat transfer. The fourmodifications of Figs. 1 to 18 and Figs-25 to 28 are shown not for thepurpose of describing all possible varieties of construction; many moremodifications are possible, and thus v 7 .they should not be consideredas limitations in any way except as set forth in the claims.

By comparing Figs. l to Figs. 6 to 9; Figs. to 14; and Figs. to 18 witheach other, it will be seen that reference character 1 which is themember, that serves to carry the electric current for heating the entireelement and for having its change of electrical resistance measured mayhave the form of a relatively thin wire as in Figs. 1-5 and in Figs.15-18, but it also may have the shape of a rod or bar of considerablecross section as in Figs. 6-9 and Figs. 10-14; the latter will usuallybe the case, if semi-conductors and especially those of the kind calledthermistors or negative temperature coefiicient resistors are used. Themember [4] 3 used for conduction of the heat also can be either in theform of a wire as in Figs. 6-18, or in the form of a bar or rod as inFigs. l-5. In Fig. 1 the electric conductor 1 is wound around a bar orrod-shaped heat conducting member 3 and they are separated by a thinlayer of insulation 4 on member 3. In Figs. 6-18, the electricalconductor is on the inside, and the heat conductor 3 on the outside. Theelectrical insulation layer 4 in Figs. 6-14 is put on the electricalconductor, while in Figs. 15-18 the electrical insulation 4 is put onthe heat conductor 3. The insulation may also be put on both members incertain applications.

These modifications bring out the following facts:

(a) It is immaterial whether the electric conductor member is on theoutside of the unit and heat conductor on the outside, or vice versa.

(b) It is immaterial whether the electric conductor is thin and the heatconductor of considerable cross section, or vice versa.

(c) The layer of electrical insulation may be on either of the twomembers or on both.

(d) Usually the thinner of the two members is wound around the one oflarger cross section, irregardless of which is the conductor forelectricity and which the heat conductor. However, that is not necessaryeither, as shown in Figs. 15-18 where a heat conductor of larger crosssection is wound around a thinner electrical conductor. In certainapplications the electrical and heat conductor are intertwined with eachother. The selection of these modified forms depends on the particularapplication and the convenience of construction. The features common toall of the several forms are recited concisely in the claims. In eachinstance a mounting is provided for the composite conductors in the formof cylindrical end members of insulation material shown at 5. The mannerof mounting the composite conductors in the end members 5 differs in thedifferent forms of the device. In Figs. l-5 the end member 5 is axiallyapertured to receive the terminals 2. In this form of our invention theends of the wire 3 are countersunk into short cylindrical blocks ofinsulation material 6 as shown at 7. The terminals 2 are countersunkinto the opposite ends of the blocks 6 as shown at 8. The blocks 6 areapertured at 9 to provide a passage for the end of the conductor whichconnects to the terminal 2. The structure at each end of the device issymmetrical so that we have described but one terminating end in detail.

In the form of our invention shown in Figs. 6-9, the end members ofinsulation material are shown at 5' recessed at 10 for centering the endconvolutions of the heat conducting member 3. The end members 5 areaxially recessed to center the terminals 2 that extend therethrough. Theterminals 2 are internally recessed at 11 to receive the ends of thewire or conductor 1.

In the form of our invention shown in Figs. 10-14 the conductor 1 isshaped as a thin bar or strip, the opposite ends of which are secured interminal members 2" that project through supporting end members ofinsulation material shown at 5 In the form of our invention shown inFigs. 15-18, the end members are shown at 5" mounting terminals 2" inmuch the .same manner as in Figs. 6-9, except that the recess at 10' isenlarged to receive and support the end convolutions of the insulatedcovering of the conductor 3.

One of the big shortcomings of straight wire and cylindrical coildetector filaments, is the fact that they are very much dependent ontheir position in space as well as with respect to the walls of theirhousing; a displacement of the detector element in the housing or of theentire unit including the housing to the extent of only a few degrees inany direction from the original position, may change the indication veryconsiderably; these changes are indeed so big that such detectorelements were used to determine the angular location of a body carryingthem in space, or to find the direction of a wind or air current. Thisfact is especially annoying when the detector unit is to be used in aportable anemometer, since it is diflicult in actual use in a mine, orin a ventilation duct always to orient the detector so that it is facingthe air current in a definite position with an accuracy of better than4-5 angular degrees.

To overcome this difliculty, in an earlier form of an anemometer whichwe developed, we used a detector element of a spherical shape; a spherebeing a body of an absolute geometrical symmetry, and one in which theconvection current as well as the thermoconductivity losses to the wallswhich are responsible for the changes caused by orientation are notchanged by any angular displacement of the element. However, a detectorelement of spherical shape proved diflicult and expensive to manufacturewith suflicient accuracy and, therefore, another basic principle wasadopted, which not only gives sufficient independence from orientation,but also aflords greater sensitivity for the measurement of low velocityair currents. Let us first make reference to a detector element as shownin Fig. 21, consisting of three equal lengths of resistance wire bent atright angles to each other, in space, so that when one of them OX is inthe direction of the X axis of a rectangular system of spacecoordinates, the other length OZ is in the direction of the Z axis, andthe third ZY in the direction of the Y axis; to simplify theexplanation, let us assume that this detector is placed in a movingfluid in such a way that the fluid moves in a direction parallel to YZor the coordinate YO. Any rotation of the unit around the YO axis willproduce no change whatsoever in the cooling conditions. The amounts ofcooling produced by the fluid flow on OZ and 0X are equal. Now let theunit be turned around, for example, the axis 0X by an angle 0:; nothingwill change in the cooling conditions of OX, but YZ will no longer beparallel to the flow and it will be cooled less; on the other hand OZ,which was before perpendicular to the flow direction, which produces aminimum of cooling, will now be turned at an equal angle to the wind,and its cooling will be increased. But this increase and this decreaseare substantially equal, and thus again there will be no change in thecooling of the entire detector unit. It can be shown, and was confirmedby our tests, that a detector unit of this construction will beindependent of any orientation in space to the same extent as one ofspherical shape, even though no special care is taken to place itsymmetrically with respect to the direction .of fluid flow.

However, a unit of this shape is diflicult to make sturdy enough forfield use, and also does not easily lend itself with the coactingconductor in any of the several arrangements heretofore explained.

To prove that a detector of conical shape under certain conditions alsopossesses a great deal of positional independence and to determine theconditions and limitations prevailing, let us turn to Figs. 29 to 30. Inthese views OXY represents a cross section of any of the conical unitsshown in Figs. 22 to 24 inclusive. The angle at the apex is equal 0,which we for the present shall subject only to the condition that it isless than 180. The fluid motion is horizontal, and so in the originalposition is the axis A of our conical detector unit. The cooling actionon OX as well as on OY from a velocity vector P, will be proportional toP cos 9/2, as follows from Fig. 29; thus the total cooling action willbe proportional to 2P cos 0/2. Now let us turn our detector element byan angle a with respect to its original position or to the fluid motion,which is assumed to be the same as before in every respect. Now as shownon Fig. 30 the cooling components for the same velocity vector P will beP cos (0/2-a and P cos (0/2+a) respectively. The sum of the two now isequal PEcos (0/2-u) +cos (0/2+a)] which by the trigonometrical formulafor the sum of cosines becomes equal to 2P cos 0/2 cos 0:. Comparingthis with the original total cooling effect 2P cos 0/2,-we see that thedifference is 2P cos 0/2 (1-cos a). From trigonometrical tables we findthat (1cos a) will become equal to .05 [and] or only when it reaches 18;thus theoretically our detector element may be turned as much as 18without affecting the indication by more than 5%. If we set 2% as thelimit of permissible error, instead of 5%, a is equal 11, which is anangmlar deviation still easy enough to observe by eye. In actualexperimentsespecially for units built according to the prescriptionsgiven hereinafter for our preferred embodiment of theinvention-conditions in this respect are still somewhat better, due tothe additional symmetry afforded by the fluid passing between thewindings through the inside of the cone; according to tests on theseunits, it takes angular deviations over 15 to introduce errors of morethan 2%.

It may be seen by observing Fig. 30, that the maximum feasible value ofangle a, is a=0/ 2; it follows that if we specify that it should bepossible to turn our detector unit by about 20 degrees without abruptlyintroducing a large error, the cone must have an apex 0 of at least 40;if a cone with this apex were turned more than 20 the cooling of theside OX, will rapidly decrease from its maximum value, which it hadreached when OX became parallel to the wind direction when 0/2a 0. Forsimilar reasons it is not desirable to make the apex of an angle 0 morethan 1804,0=l40.

It should be noted, that while with the spherical and three-coordinateshapes of detector units, the original position is immaterial, a conicalshaped detector unit when used for fluid flow measurements for bestresults should originally be placed with the axis of the conesubstantially parallel to the direction of the fluid flow; exception tothis are detectors shaped in the form of double cones with a common baseas in Fig. 23, which may be placed originally in any position; also,when the detector elements are not used for flow measurements, but todetermine temperatures, thermoconductivity or other thermal exchangevalues in a stationary medium any of the cone shaped elements does notneed definite orientation. For thermoconductivity detectors and otherthermal exchange units where the surrounding medium is stationary or theeffect of its motion is negligible, all the modifications shown in Figs.22 to 28 will give good results. For anemometer or fluid flow detectorswe have found the embodiment shown in Figs. 25 to 28 preferable toothers.

Although any of the basic composite elements shown in Figs. 1-18 mightbe used, in the embodiment shown 10 on Figs. 25-28 the basic element ofFig. 1 with some minor changes is used; the terminals 2 to which theresistance wire 1 is electrically connected instead of being solid, arenow tube shaped as shown at 2" the part with high thermoconductivity isnow extended somewhat beyond the windings of the resistance wire 1inside of the tubular terminals 2" and surrounded by a substantial layerof a plastic insulating and cementing material 4" to form a mechanicallystrong and well insulated-thermally as well as electricallyjoint withthe terminals 2" and base plates 5 The composite electric and heatconductor is wound into the shape of a cone with an angle about 60. Togive the unit the highest possible sensitivity, the consecutive turns ofthe spiral which is the projection on a plane perpendicular to the conesaxis (and thus also to the flow), must be spaced-as shown on Fig. 28--insuch a way that the width of the empty interspaces in any directionperpendicular to the flow is equal or larger than the overall diameterof the composite conductor; then, fluids, moving at not too highvelocities, will travel not only along the outside of the detectorelement, but also through the inside and between the interstices and,thus, the cooling action will be greatly increased. As long as thiscondition is fulfilled, the maximum number of turns is limited only bythe size of the completed unit. The minimum obviously is one completeturn.

The number of actual turns in a completed detector unit, will depend onthe size of the composite electricheat conductor used, and the latterwill in turn depend on the sizes of the heat and electrical conductorsused. One of our actual detector units of this type consists of twoinches of a .002 diameter nickel alloy wire, known under the trade nameof Hytemco, wound around a 2" length of number 30 copper wire withFormex insulation; this assembly is wound into the shape of a conicalunit of about 60 apex, and Ms" side and base diameter; there are aboutthree spiral convolutions. The entire assembly is coated with a thinlayer of Glyptal, baked on at C., which makes it a single unit of greatstrength and durability, yet also of high sensitivity. This unit whenused as an anemometer, gives suflicient sensitivity to cover a range offrom 10 feet to 150 feet per minute of air velocity with less than 3volts across the Wheatstone bridge circuit of which it is a part. Whenused as a flow indicator, a full scale deflection [of] from 100 cubiccentimeters per minute of air flow is easily obtained.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. A sensitive element for the measurement of heat exchange with asurrounding medium comprising: a flexible member with a high temperaturecoefficient of electrical resistance, two terminals electroconductivelyjoined to the ends of said member for leading electric current to andfrom said member, a flexible member of high thermal conductivity, a thinlayer of electric insulation on one of said members, one of said membersbeing tightly coiled around the other member to form a substantiallysolid composite element of high thermal and relatively low electricalconductivity and said composite element being wound in the shape of aconical coil.

2. A sensitive element for the measurement of heat exchange with asurrounding medium comprising: a flexible member with a high temperaturecoeflicient of electrical resistance, two terminals electroconductivelyjoined to the ends of said member for leading electric current to andfrom said member, a flexible member of high thermal conductivity, a thinlayer of electric insulation on one of said members, one of said membersbeing tightly coiled around the other member to form a substantiallysolid composite element of high thermal and relatively low electricalconductivity and said composite element being wound in the shape of aconical coil with an apex of between 40 and angular degrees.

3. A sensitive element for the measurement of heat ex change with asurrounding medium comprising: a flex= ible member with a hightemperature coeflicientof electrical resistance, two terminalselectroconductively joined to the ends of said member for leadingelectric current to and from said member, a flexible member of highthermal conductivity, a thin layer of electric insulation on one of saidmembers, one of said members being tightly coiled around the othermember to form a substantially solid composite element of high thermaland relatively low electrical conductivity and wound in the shape of twocoaxial cones with a common base and with apexes of 90 degree angles.

4. A sensitive element for the measurement of heat exchange with asurrounding medium comprising: a flexible member with a high temperaturecoeflicient of electrical resistance, two terminals electroconductivelyjoined to .the ends of said member for leading electric currentto andfrom said member, a flexible member of high thermal conductivity, a thinlayer of electric insulation on one of said members, one of said membersbeing tightly coiled around the other member to form a substantiallysolid composite element of high thermal and relatively low electricalconductivity and wound in the shape of two coaxial cones with a commonbase and substantiallyequal apexes.

5. A sensitive element for the measurement of heat exchange with asurrounding medium, comprising an electric resistance wire memberconsisting of three electrically continuous parts of substantially equallength, surface area and thermal and electrical resistancecharacteristics, said parts being bent at right angles to each other inspace, and being disposed substantially parallel to the three directionsof a rectangular system of space coordinates.

6. A sensitive element for the measurement of heat exchange with asurrounding medium comprising: a flexible member with a high temperaturecoeflicient of electrical resistance, two terminals electroconductivelyjoined to the ends of said member for leading electric current to andfrom said member, a flexible member of high thermal conductivity, a thinlayer of electric insulation. [of] on one of said members, one of saidmembers being tightly coiled around the other member to form asubstantially solid composite element of high thermal and relatively lowelectrical conductivity, said composite element being subdivided alongits length into three parts of substantially equal length, surface areaand thermal and electrical characteristics, said parts bent at rightangles to each other and substantially in parallel with threerectangular space coordinates.

7. A sensitive element for the measurement of fluid flow and velocityincluding a flexible combined intertwisted electrical and thermalconductor member shaped in the form of a conical coil having consecutiveturns so spaced that their projection on a plane perpendicular to thecone axis forms a flexible spiral with interstices not smaller than thethickness of the conductor.

8; A sensitive element for the measurement of fluid flow and velocitycomprising a flexible member having a high temperature coeflicient ofelectrical resistance, a terminal electroconductively joined to each endof said member for leading electric current to and from said member, aflexible member of high thermal conductivity, a relatively thin layer ofelectric insulation on one of said members, one of said members beingtightly coiled around the other to form a substantially solid compositeelement, said composite element being wound in the shape of a conicalcoil, with an apex of approximately angular degrees and havingconsecutive turns so spaced that for a fluid moving in the direction ofthe axis of said cone, the free passage between the convolutions at.

least equals the thickness of the composite element.

9. A sensitive element for the measurement of heat exchange with amedium surrounding it, comprising a member with a high temperaturecoefficient of electrical resistance, a flexible member of high thermalconductivity, electrical insulation on one of said members thin enoughto permit substantially unimpeded transmittance of heat, said member ofhigh thermal conductivity being tightly wound around said first-namedmember to increase the surface area of the element and to control theamount of heat energy exchanged.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS 1,156,638 Simmons Oct. 12, 1915 1,222,492 ThomasApr. 10, 1917 1,304,687 Kahn May 27, 1919 12,321,846 Obermaier June 15,1943

